For many years now, telecommunications carriers have been deploying packet-switched networks in place of, or overlaid upon, circuit-switched networks for reasons of efficiency and economy.
Link Aggregation (IEEE 802.3ad), also known as MultiLink Trunking (MLT) is a standardised way of bonding together groups of Ethernet links, called a Link Aggregation Group (LAG), to make them appear as a single higher-capacity link. MLT simplifies network operations by reducing the number of links that need to be managed and by improving redundancy. It is implemented by many vendors on existing hardware. MLT has been incorporated into the IEEE 802.3 standard at clause 34. The MLT standard defines a Frame Distributor which is responsible for taking outgoing frames of data from a Medium Access Control (MAC) client and transmitting them through the set of links that form the Link Aggregation Group. IEEE 802.3 does not define an algorithm for distributing frames between links in the LAG, but simply requires that any algorithm shall not cause: (i) mis-ordering of frames that form a conversation or (ii) duplication of frames. Consequently, practical systems using MLT rely on a proprietary hashing algorithm to allocate frames between the links that make up a LAG and currently each vendor has their own implementation.
Nortel Networks implements MultiLink Trunking in their ERS8000 (Ethernet Routing Switch) Series of 10 Gigabit Ethernet (10 GE) switches. The hashing algorithm implemented by this product allocates frames to links in a LAG by an operation based on part of the source and destination MAC addresses of an Ethernet frame or, if an Ethernet frame is carrying an IP packet, uses the IP addresses of packets. Another vendor—Cisco-implemented MLT in a protocol called EtherChannel™. Frame distribution is based on a MAC address (Layer 2), an IP address (Layer 3) or a port number (Layer 4). Distribution can be based solely on the source address, destination address, or both source and destination address. Both of these existing algorithms are optimised for conventional Ethernet traffic. These conventional algorithms rely on the diversity of source and destination MAC addresses that are seen in arriving frames to spread the arriving traffic across the links that make up the Link Aggregation Group.
MAC-in-MAC (M-in-M, IEEE 802.1ah, Provider Backbone Bridges) is a forthcoming standard which allows customer Ethernet traffic to be carried transparently across a carrier Ethernet network, by encapsulating the customer frame inside a second Ethernet header. When the existing hashing algorithms outlined above are applied to MAC-in-MAC traffic they result in a poor distribution of frames between links in a LAG because the source and destination MAC addresses operated upon by those algorithms have a very limited address space, e.g. the several hundred addresses of the carrier network compared to many thousands of addresses for conventional Ethernet.
The present invention seeks to improve the distribution of frames across links in a link aggregation scheme.